This invention relates to a current source inverter. More particularly, it relates to a current source inverter with a protective device which suppresses an overvoltage appearing in case of a power failure of an a.c. power source for supplying a.c. power to the current source inverter.
A current source inverter is constructed of a converter for changing alternating current into direct current, an inverter for changing the d.c. output of the converter into alternating current, and a reactor disposed in a d.c. circuit between the converter and the inverter. The reactor functions as a current source. Such current source inverters are disclosed in, and known from, for example, U.S. Pat. No. 3,796,935 and U.S. Ser. No. 716,205.
In the current source inverters, the d.c. circuit is provided with the reactor of large reactance. Therefore, when an accident as suddenly cuts off the circuit through which the direct current flows occurs, a great voltage is generated owing to energy held by the reactor. This voltage results in breaking down the converter. In case where an input power source of the converter has gone "off," the converter ceases its operation and therefore acts similarly to abruptly cutting off the circuit through which the direct current flows. In this case, accordingly, the same problem is involved.
In order to solve this problem, the power failure of the input power source is detected, and both terminals of the reactor are substantially short-circuited at the time of the power failure, thereby to prevent the high voltage from being generated in the reactor. However, when the period of time for detecting the power failure of the a.c. input power source becomes long, the generation of the high voltage cannot be satisfactorily prevented even by the aforecited protective operation at the time of power failure.